VI. Analysis

4. 2MASS Photometric System

a. Absolute Calibration

2MASS photometry is quoted in the internal 2MASS photometric system.
Conversions between the 2MASS and other popular near-infrared photometric
systems were derived empirically by
Carpenter et al. (2001, AJ, 121, 2851) using point source photometry
from the 2MASS Second Incremental Data Release. Those transformations
have been updated using the All-Sky Release PSC and are presented
in VI.4b.

2MASS Survey observations did not enable a direct measurement
of the absolute calibration of the 2MASS photometric system.
Cohen et al.
(2003, AJ, 126, 1090)
have determined photon-counting relative spectral response curves (RSRs)
for the 2MASS system by combining the transmission and QE curves provided in
Section III.1.b1.
Figures
1,
2, and
3 show these RSRs,
with peaks renormalized to unity, and including atmospheric
transmission as modeled by
PLEXUS.
These authors offer an absolute flux calibration for 2MASS based on the
calibrated spectrum of Vega from
Cohen et al. (1992 AJ, 104, 1650).
Characteristic quantum efficiency curves for the NICMOS III detectors
supplied by Rockwell were used in the computation, but have not been
measured explicitly for each device in the 2MASS cameras.

The isophotal wavelengths, bandpasses, and fluxes for zero
magnitude for the three 2MASS bands from
Cohen et al. (2003) are listed
in Table 1 below. Figures
21 and
22
in Section III.1.b1 show
how the 2MASS J-band system response changes
slightly with varying levels of atmospheric water vapor or atmospheric
aerosols. The J-band values in Table 1 correspond to about
5.0 mm of water vapor, but are based on PLEXUS calculations, rather than
the simpler ATRAN results shown in Figures
21 and
22 in
III.3.b1.

Figures
1,
2, and
3 present the 2MASS J, H and Ks
relative spectral response curves (RSRs),
peak-normalized to unity, derived by
Cohen et al. (2003).
As stated by these authors, these curves "are designed to be integrated
directly over stellar spectra
in Fλ form, in order to calculate synthetic photometric
magnitudes. The QE-based component was converted to yield photon-counting
RSRs by multiplying by wavelength and renormalized, as described
by Bessel (2000)."
These RSRs are consistent with the absolute calibration of 2MASS given in
Table 1 above.